On the Variability of Winds, Sea Surface Temperature, and Surface Layer Heat Content
in the Western Equatorial Pacific

Michael J. McPhaden and Stanley P. Hayes

NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington

Journal of Geophysical Research, 96, supplement, 3331-3342 (1991)
This paper is not subject to U.S. copyright. Published in 1991 by the American Geophysical
Union.

3.Wind-Related Processes Affecting SST and Surface Layer Heat Content

In this section we review the processes related to wind forcing that can affect
SST and surface layer heat content. Arguments are cast in terms of simple mixed
layer models [e.g.,
Niiler and Kraus, 1977; McPhaden,
1982], where it is assumed that the temperature and velocity are uniform
over some depth h. This is done for illustrative purposes only, since
our measurements will show that the assumption of vertical uniformity of temperature
variations is invalid in the western equatorial Pacific on the time scales we
consider. Nonetheless, the model is conceptually useful for guiding the analysis.

With the above assumption, one can write

(1)

where T/t
is the local time rate of temperature change, u is horizontal velocity,
w, is an entrainment velocity
through the base of the mixed layer at depth z = -h, and Q
is surface heat flux. The horizontal temperature gradient is denoted by T, and T
is the change in temperature at the base of the mixed layer. Water density ()
and the heat capacity of water at constant pressure (C)
are assumed constant at 10 kg m
and 3.94 × 10 J kg°C,
respectively. Only terms that can be readily related to the wind field (as discussed
below) and can be examined with our data sets have been retained in (1); other
terms, namely, lateral turbulent diffusion, vertical turbulent diffusion through
the base of the mixed layer, and penetrative solar radiation, have been neglected.
These neglected terms may also depend on wind forcing, though it is not obvious
how, with out limited data set, we can critically examine their relationship
to SST and surface layer heat content.

3.1. Turbulent Heat Exchange at the Surface

Surface heat exchange Q
is made up of longwave and shortwave radiative fluxes plus sensible and latent
turbulent heat fluxes. Bulk parameterizations of the turbulent heat fluxes are
given by

(2a)
(2b)

where |U| is wind speed, q
is an air-sea specific humidity difference, T
is an air-sea temperature difference, C
is a turbulent exchange coefficient,
(= 1.2 kg m) is air density, and L
(= 2440 J g) is the latent heat of evaporation.
Wind speed dependence is explicit in these expressions; in addition, exchange
coefficients may depend on wind speed [e.g.,
Large and Pond, 1982]. Latent heat fluxes with monthly mean values
up to 100 W m are generally the larger of
the two turbulent fluxes in the western equatorial Pacific [e.g.,
Reed, 1985; Liu,
1988], so that most discussions of turbulent exchange focus on this
component alone.

Meyers
et al. [1986], using shipboard data, and Liu
[1988], using satellite data, have shown that during the 1982-1983 ENSO,
latent heat flux variations on monthly time scales were highly correlated with
wind speed variations in the western equatorial Pacific. However, these studies
differed in their conclusion about the relative importance of latent heat fluxes
in the surface temperature balance. Liu found little correlation between these
fluxes and SST variability along the equator; in contrast, Meyers et al. suggested
that a net 1°C cooling of the western Pacific warm pool during 1982-1983 resulted
from large latent heat fluxes associated with two episodes of strong equatorward
meridional winds during mid-1982 (southern winter) and early 1983 (northern
winter). The reason for these conflicting conclusions is unclear.

In a study that utilized many of the same time series data presented here,
McPhaden
et al. [1990] noted that high wind speed variations on weekly to monthly
time scales during the 1986-1987 ENSO were associated with cool SST near the
equator. This led to a suggestion that there existed a relationship between
variations in latent heat flux and surface cooling on these time scales. However,
McPhaden et al. did not quantify the suggested relationship between SST and
wind speed in support of this hypothesis.

3.2. Vertical Advection and Entrainment

Vertical advection and entrainment are related but distinct processes involved
in changing surface layer heat content and SST. Their combined effect during
periods of easterly winds is often referred to as equatorial upwelling. Entrainment
velocity w can be written
as

(3)

where wis vertical
velocity at the base of the surface layer and h/t
is the local time rate of change of the surface layer thickness. The inequality
implies that in a stably stratified ocean, entrainment can only cool the surface.
The velocity w is sometimes
inferred from the vertical displacement of isotherms in the thermocline (assuming
the effects of turbulent diffusion and lateral advection can be ignored). Processes
contributing to ware
tides; small-scale, high-frequency internal waves; equatorial waves; and Ekman
pumping due to local wind-driven variations in horizontal mass convergence and
divergence. When Ekman dynamics are operative, won the equator will be proportional to the local zonal wind stress

(4)

where C is a drag coefficient
and U is zonal wind speed.

In the limit where h is a constant, entrainment velocity is just the
vertical velocity at the base of the mixed layer. Under these circumstances,
thermocline displacements (given the assumptions above) and SST would be in
phase and highly correlated. Conversely, in the limit where the base of the
mixed layer is a material surface (that is, w=
0), the mixed layer depth would change coherently with changes in thermocline
depth and there would be no effect of entrainment on SST.

Entrainment can also be parameterized in terms of the surface layer turbulent
energy balance [Niiler
and Kraus, 1977; McPhaden,
1982]. Two sources of turbulent energy production directly related to
the wind field are wind work (proportional to wind speed cubed) and free convection
due to heat loss by evaporation and conduction (proportional to wind speed).
The role of free convection cannot be evaluated without taking into account
surface buoyancy fluxes caused by radiative heating and precipitation. On the
other hand, the effect of wind work, which is always a source of turbulent energy
production, can be expressed by

(5)

where is
the density jump across the base of the mixed layer, g is gravity, and
m is an empirically determined efficiency factor [Niiler
and Kraus, 1977]. Note that the efficiency of entrainment decreases
as layer depth increases because, for a given wind speed, more of the water
column has to be energized with turbulence. Other sources of turbulent energy
production may enhance this entrainment (for example, production by vertical
shear instability at the base of the mixed layer), but they can not be reliably
estimated from our data set.

The role of entrainment mixing in the heat balance of the western equatorial
Pacific was discussed by Meyers
et al. [1986], who concluded that its effect on SST during the 1982-1983
ENSO was relatively small. McPhaden
et al. [1990] found evidence for equatorial upwelling (that is, upward
advection and entrainment) during a period of strong easterly winds at 165°E
in early 1988. However, based on the incoherence of SST and thermocline depth
variations (as measured by the depth of the 20°C isotherm over a period of 2.5
years during 1986-1988), they inferred that, in general entrainment of cold
waters from the thermocline was not important in determining SST variability
in the western equatorial Pacific. They suggested that this was owing to the
relatively thick (50-100 m) surface layer and the fact that it was not always
completely mixed with regard to temperature or salinity. Lukas
and Lindstrom [this issue] and Godfrey
and Lindstrom [1989] have hypothesized that the existence of a salt-stratified
"barrier layer" in the temperature mixed layer inhibits entrainment
in the western equatorial Pacific except during infrequent, large-amplitude
westerly wind bursts. However, in none of these studies was the relationship
between SST, surface layer heat content, and wind work according to (5)
examined.

3.3. Lateral Advection

Another process that can cause SST to change is lateral advection. This is
represented by the term u · T
in (1), which assumes slablike flow over the layer
depth h. Whereas this is not strictly valid in the warm-pool region of
the western Pacific, a significant component of flow is depth coherent in the
surface layer [e.g.,
McPhaden et al., 1988]. Moreover, for a purely advective balance,